Replacement gate process for FinFET

ABSTRACT

A method of forming a semiconductor device includes etching a substrate to form two first trenches separated by a fin; filling the two first trenches with an isolation layer; and depositing a dielectric layer over the fin and the isolation layer. The method further includes forming a second trench in the dielectric layer over a channel region of the semiconductor device, the second trench exposing the isolation layer. The method further includes etching the isolation layer through the second trench to expose an upper portion of the fin in the channel region of the semiconductor device, and forming a dummy gate in the second trench over the isolation layer and engaging the upper portion of the fin.

BACKGROUND

The semiconductor integrated circuit (IC) industry has experienced exponential growth. Technological advances in IC materials and design have produced generations of ICs where each generation has smaller and more complex circuits than the previous generation. In the course of IC evolution, functional density (i.e., the number of interconnected devices per chip area) has generally increased while geometry size (i.e., the smallest component (or line) that can be created using a fabrication process) has decreased. This scaling down process generally provides benefits by increasing production efficiency and lowering associated costs. Such scaling down has also increased the complexity of processing and manufacturing ICs and, for these advancements to be realized, similar developments in IC processing and manufacturing are needed.

For example, multi-gate devices have been introduced in an effort to improve gate control by increasing gate-channel coupling, reduce OFF-state current, and reduce short-channel effects (SCEs). One type of multi-gate devices is FinFETs—transistors with a fin-like semiconductor channel (“fin”) and a gate electrode engaging the fin on two or three sides thereof. A further advancement in FinFET is the replacement of the typically poly silicon gate electrode with a metal gate electrode to improve device performance. This is termed a “replacement gate” or “gate last” process in which a dummy gate (e.g., a poly silicon gate) is fabricated over the fin and is replaced with a final gate stack (e.g., a metal gate) in later fabrication steps. This allows for reduced number of subsequent processes, including high temperature processing, that is performed after the formation of the final gate stack. However, there are challenges to implementing such IC fabrication processes, especially with scaled down IC features in advanced process nodes. One challenge is that after the fin is formed and before the dummy gate is formed, the fin may suffer from buckling and collapsing during cleaning and drying processes due to its high aspect ratio (a ratio of fin height over fin width).

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is emphasized that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion.

FIGS. 1A, 1B, 1C, and 1D are flow charts of a method of forming a semiconductor device according to various aspects of the present disclosure.

FIGS. 2A, 2B, 2C, 2D, 2E, 2F, 2G, 2H, 2I, 2J, 2K, 2L, 2M, 2N, 2O, and 2P are perspective views of a portion of a semiconductor device in various fabrication stages according to the method in FIGS. 1A-1D, in accordance to an embodiment.

FIGS. 3A, 3B, 3C, 3D, 3E, and 3F are perspective views of a portion of a semiconductor device in various fabrication stages according to the method in FIGS. 1A-1D, in accordance to another embodiment.

FIG. 4 is a perspective view of a portion of a semiconductor device fabricated with the method in FIGS. 1A-1D, in accordance to an embodiment.

FIGS. 5A and 5B are cross-sectional views of portions of a semiconductor device fabricated with the method in FIGS. 1A-1D, in accordance to an embodiment.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, or examples, for implementing different features of the provided subject matter. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper” and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly.

The present disclosure is generally related to methods for semiconductor device fabrication, and more particularly to methods of forming FinFETs using a replacement gate process. In a typical FinFET replacement gate process, a plurality of fins is formed over a substrate through patterning and cleaning processes. Then, a dummy gate is formed over the substrate and engaging the plurality of fins. After some fabrication steps, such as the formation of doped source/drain (S/D) features, the dummy gate is replaced by an actual gate including metal layers. One problem with such a replacement gate process is that the plurality of fins is left free-standing on the substrate through one or more cleaning processes before the formation of the dummy gate. The one or more cleaning processes may include a wet cleaning process with a solvent having high surface tension followed by a spin drying process. Due to the lack of support and protection, the free-standing fins may buckle or collapse during the one or more cleaning processes. This problem is more severe with fins that have a high aspect ratio (e.g., an aspect ratio greater than 9). Embodiments of the present disclosure provide solutions to the above issue. Although the present disclosure discusses embodiments in terms of FinFETs, the inventive concepts can be applied to other type of devices, including other types of multi-gate devices, such as horizontal gate-all-around devices having a nanowire channel. One of ordinary skill may recognize other examples of semiconductor devices that may benefit from aspects of the present disclosure.

FIGS. 1A, 1B, 1C, and 1D show flow charts of a method 10 of forming a semiconductor device 100 using a replacement gate process, according to various aspects of the present disclosure. The method 10 is merely an example, and is not intended to limit the present disclosure beyond what is explicitly recited in the claims. Additional operations can be provided before, during, and after the method 10, and some operations described can be replaced, eliminated, or moved around for additional embodiments of the method. The method 10 is described below in conjunction with FIGS. 2A-2P and 3A-3F, which show perspective views of a portion of the semiconductor device 100 in intermediate stages of fabrication.

At operation 12, the method 10 (FIG. 1A) receives a substrate 102. Referring to FIG. 2A, in embodiments, the substrate 102 may be a semiconductor substrate such as a silicon wafer. The substrate 102 may also include other semiconductors such as germanium, a compound semiconductor such as silicon carbide, gallium arsenide, gallium phosphide, indium phosphide, indium arsenide, and/or indium antimonide, an alloy semiconductor such as GaAsP, AlInAs, AlGaAs, InGaAs, GaInP, and/or GaInAsP, or combinations thereof. Further, the substrate 102 may optionally include epitaxial layers, be strained for performance enhancement, include a silicon-on-insulator structure, and/or have other suitable enhancement features.

At operation 14, the method 10 (FIG. 1A) etches the substrate 102 to form trenches 104 therein. Referring to FIG. 2B, portions of the substrate 102 are removed to form the trenches 104, and one or more fins 106 are thereby formed with remaining portions of the substrate 102. The one or more fins 106 extend from a bottom portion of the substrate 102, and interpose the trenches 104. In an embodiment, operation 14 includes a patterning process. For example, the patterning process forms a masking element over the substrate 102 through a photolithography process. The photolithography process may include forming a photoresist (or resist) over the substrate 102, exposing the resist to a pattern that defines the shapes for the fins 106 (or the trenches 104) from a top view, performing post-exposure bake processes, and developing the resist to form the masking element. The operation 14 further includes an etching process where the substrate 102 is etched through the masking element to form the trenches 104. The etching processes may include one or more dry etching processes, wet etching processes, and other suitable etching techniques. For example, a dry etching process may implement an oxygen-containing gas, a fluorine-containing gas (e.g., CF₄, SF₆, CH₂F₂, CHF₃, and/or C₂F₆), a chlorine-containing gas (e.g., Cl₂, CHCl₃, CCl₄, and/or BCl₃), a bromine-containing gas (e.g., HBr and/or CHBR₃), an iodine-containing gas, other suitable gases and/or plasmas, and/or combinations thereof. For example, a wet etching process may comprise etching in diluted hydrofluoric acid (DHF); potassium hydroxide (KOH) solution; ammonia; a solution containing hydrofluoric acid (HF), nitric acid (HNO₃), and/or acetic acid (CH₃COOH); or other suitable wet etchant. The one or more etching processes form the trenches 104 in the substrate 102, leaving portions of the un-etched substrate 102 as the fins 106. The masking element is subsequently removed, for example, by a stripping process. In an embodiment, the fins 106 have a high aspect ratio, which is the ratio between the height (along the “z” direction) and the width (along the “x” direction) of the fins 106. For example, the aspect ratio of the fins 106 may exceed 9.

At operation 16, the method 10 (FIG. 1A) fills the trenches 104 with an isolation layer 108. Referring to FIG. 2C, the isolation layer 108 includes a dielectric material, such as silicon oxide, which electrically isolate the fins 106 from each other. The isolation layer 108 may be formed by chemical vapor deposition (CVD), plasma enhanced CVD (PECVD), physical vapor deposition (PVD), thermal oxidation, or other techniques. In an embodiment, after the isolation layer 108 is deposited, a chemical mechanical planarization (CMP) process is performed to planarize a top surface of the isolation layer 108 and to expose top surfaces of the fins 106. As a result, the top surface of the isolation layer 108 and the top surfaces of the fins 106 are co-planar.

At operation 18, the method 10 (FIG. 1A) optionally recesses the fins 106 and grows one or more epitaxial layers as an upper portion of the fins 106. This may involve multiple steps as shown in FIGS. 2D and 2E. Referring to FIG. 2D, the device 100 is etched in a selective etching process where the fins 106 are etched while the isolation layer 108 remains substantially unchanged. As a result, trenches 110 are formed in the device 100. Various portions of the isolation layer 108 act as sidewalls of the trenches 110. The remaining portions of the fins 106 are labeled as 106 a, which act as the floor of the trenches 110. The etching process may include dry etching, wet etching, or other suitable etching techniques.

Referring to FIG. 2E, epitaxial features 106 b and 106 c are grown in the trenches 110 atop the original fins 106 a. The features 106 b and 106 c and the original fins 106 a make up the fins 106 for the subsequent fabrication stages. In an embodiment, the feature 106 b may include one or more semiconductor layers, each of which may be grown by a molecular beam epitaxy (MBE) process, a chemical vapor deposition (CVD) process such as a metal organic CVD (MOCVD) process, and/or other suitable epitaxial growth processes. Each semiconductor layer of the feature 106 b may include silicon, germanium, a compound semiconductor, or an alloy semiconductor, and may be doped or undoped. The feature 106 c is an epitaxial hardmask layer in the present embodiment, and may be omitted in an alternative embodiment. The feature 106 c has a slower etching rate than the feature 106 b and can protect the semiconductor layer(s) of the feature 106 b in subsequent processes. In an embodiment, after the epitaxial features 106 b and 106 c are grown, a CMP process is performed to planarize top surfaces of the isolation layer 108 and the features 106 c/106 b. The operation 18 is optional—in an alternative embodiment, the steps shown in FIGS. 2D and 2E are not performed.

At operation 20, the method 10 (FIG. 1A) deposits a dielectric layer 112 over the isolation layer 108 and the fins 106. Referring to FIG. 2F, the dielectric layer 112 may include materials such as tetraethylorthosilicate oxide, un-doped silicate glass, or doped silicon oxide such as borophosphosilicate glass, fused silica glass, phosphosilicate glass, boron doped silicon glass, and/or other suitable dielectric materials. The dielectric layer 112 may be deposited by a PECVD process or other suitable deposition techniques. In an embodiment, the dielectric layer 112 will be completely removed in subsequent fabrication stages. Therefore it is also called a dummy dielectric layer 112.

At operation 22, the method 10 (FIG. 1A) forms trenches 114 in the dielectric layer 112. Referring to FIG. 2G, the trenches 114 are formed over channel regions 116 of the device 100. The channel regions 116 correspond to the regions of transistor channels where gate electrodes are to be formed. In an embodiment, operation 22 includes a photolithography process and an etching process. For example, the photolithography process forms a masking element over the dielectric layer 112. Then, the dielectric layer 112 is etched through the masking element using a dry etching process or a wet etching process that selectively removes the material(s) of the dielectric layer 112 but not the isolation layer 108 and the fins 106. As a result, the top surfaces of the isolation layer 108 and the fins 106 are exposed through the trenches 114. The masking element may be subsequently removed.

At operation 24, the method 10 (FIG. 1A) etches the isolation layer 108 through the trenches 114 to expose upper portions of the fins 106. Referring to FIG. 2H, the trenches 114 extend into the isolation layer 108 such that the upper portions of the fins 106 reach a desired height FH. Further, the epitaxial feature 106 c (FIG. 2E) are removed in the channel regions 116 during the etching process. In an embodiment, the etching process is a dry etching process that is tuned to selectively remove the isolation layer 108 while the dielectric layer 112 and the fins 106 (at least the features 106 b and 106 a) remain substantially unchanged. In an embodiment, the fins 106 may be cleaned, for example, using a wet cleaning solution followed by a spin drying process. Since the fins 106 are supported by the isolation layer 108 in various portions, they do not suffer buckling or collapsing issues as discussed earlier with respect to traditional replacement gate processes. In an embodiment, the upper portions of the fins 106 may be passivized to form a thin passivation layer.

At operation 26, the method 10 (FIG. 1B) forms dummy gates 117 (see FIGS. 2L and 3C) in the trenches 114 and engaging the upper portions of the fins 106. At operation 28, the method 10 (FIG. 1B) removes the dielectric layer 112 from the device 100 (or at least from S/D regions of the device 100). Operations 26 and 28 may be executed in different orders in various embodiments. Operation 26 further involves multiple steps. In the following, operations 26 and 28 are discussed using two embodiments of the present disclosure. The first embodiment is illustrated in FIG. 1C in conjunction with FIGS. 2I-2M. The second embodiment is illustrated in FIG. 1D in conjunction with FIGS. 3A-3C. One of ordinary skill may recognize other embodiments from aspects of the present disclosure.

In the first embodiment, operation 26 follows operation 24 and includes operations 50, 52, 54, and 56 (FIG. 1C). At operation 50, the method 10 (FIG. 1C) forms an oxide or nitride layer (e.g., silicon oxide, silicon nitride, or silicon oxynitride) over the fins 106 in the second trenches 114. The oxide or nitride layer may be formed by chemical oxidation, thermal oxidation, atomic layer deposition (ALD), chemical vapor deposition (CVD), and/or other suitable methods. In an embodiment, the oxide or nitride layer is a thin and conformal layer.

At operation 52, the method 10 (FIG. 1C) fills the trenches 114 with a poly silicon layer 118. Referring to FIG. 2I, the poly silicon layer 118 is deposited in the trenches 114 over the oxide or nitride layer (not shown) over the upper portions of the fins 106. FIG. 2I further shows that the poly silicon layer 118 overfills the trenches 114 and is disposed over the top surfaces of the dielectric layer 112. In an embodiment, the poly silicon layer 118 is formed by low-pressure chemical vapor deposition (LPCVD), plasma-enhanced CVD (PECVD), or other suitable deposition processes.

At operation 54, the method 10 (FIG. 1C) partially recesses the poly silicon layer 118 into the trenches 114. Referring to FIG. 2J, the poly silicon layer 118 is recessed such that a top surface 118′ of the poly silicon layer 118 is below a top surface 112′ of the dielectric layer 112, but above a top surface 108′ of the isolation layer 108. In an embodiment, the depth of the trenches 114 (from 118′ to 112′ along the “z” direction) is about one third of the height of the dummy gate 117 (see FIG. 2L). In the present embodiment, this depth is controlled by timer for the etching process in the operation 54.

At operation 56, the method 10 (FIG. 1C) forms a hard mask layer 120 in the trenches 114. Referring to FIG. 2K, the hard mask layer 120 is deposited as a blanket layer over the device 100 and overfilling the trenches 114. In embodiments, the hard mask layer 120 provides etching selectivity with respect to the dielectric layer 112 and/or the isolation layer 108. In an embodiment, the hard mask layer 120 includes a nitride, such as silicon nitride, silicon oxynitride, and silicon oxycarbon nitride. In an alternative embodiment, the hard mask layer 120 includes other types of dielectric layer suitable for hard mask purposes, such as silicon oxide. The hard mask layer 120 may be formed by chemical oxidation, thermal oxidation, ALD, CVD, and/or other suitable techniques. Referring to FIG. 2L, a CMP process is performed to remove the hard mask layer 120 outside the trenches 114 and to planarize a top surface of the device 100. As shown in FIG. 2L, the dummy gate 117 includes the hard mask layer 120, the poly silicon layer 118, and the oxide or nitride layer (not shown) underneath the poly silicon layer 118 in this embodiment.

Following operation 56, the method 10 (FIG. 1C) removes the dielectric layer 112 in operation 28. Referring to FIGS. 2L and 2M, the dielectric layer 112 is removed from at least source/drain (S/D) regions 122 of the device 100. The dielectric layer 112 may be removed by an etching process tuned to selectively remove the dielectric layer 112 while the dummy gate 117, the isolation layer 108, and the fins 106 remain substantially unchanged.

In the second embodiment, operation 26 follows operation 28 and includes operations 60 and 62 (FIG. 1D). Referring to FIGS. 2H and 3A, the dielectric layer 112 is removed from at least the S/D regions 122 of the device 100 while the upper portions of the fins 106 are still exposed through the trenches 114. The dielectric layer 112 may be removed by an etching process tuned to selectively remove the dielectric layer 112 while the isolation layer 108 and the fins 106 remain substantially unchanged. The upper portions of the fins 106 may be cleaned subsequently.

At operation 60, the method 10 (FIG. 1D) forms a hard mask layer 123 in the trenches 114. Referring to FIG. 3B, the hard mask layer 123 is deposited over the isolation layer 108 and engaging the upper portions of the fins 106. In the present embodiment, the hard mask layer 123 overfills the trenches 114 and is disposed over the top surfaces of the isolation layer 108. In embodiments, the hard mask layer 123 provides etching selectivity with respect to the isolation layer 108. In an embodiment, the hard mask layer 123 includes a nitride, such as silicon nitride, silicon oxynitride, and silicon oxycarbon nitride. In an alternative embodiment, the hard mask layer 123 includes other types of dielectric layer suitable for hard mask purposes, such as silicon oxide. The hard mask layer 123 may be formed by chemical oxidation, thermal oxidation, ALD, CVD, and/or other suitable techniques. At operation 62, the method 10 (FIG. 1D) performs a CMP process to remove the hard mask layer 123 outside the trenches 114 and to planarize a top surface of the device 100 (FIG. 3C). As shown in FIG. 3C, the dummy gate 117 includes the hard mask layer 123 in this embodiment.

At operation 30, the method 10 (FIG. 1B) recesses the isolation layer 108 in the S/D regions 122. Referring to FIGS. 2M-2N and FIGS. 3C-3D, with the dummy gate 117 covering the fins 106 in the channel regions 116, the isolation layer 108 is etched in the S/D regions 122. The etching process may include a dry etching process, a wet etching process, or other suitable etching techniques. The etching process is tuned to selectively remove the isolation layer 108 while the dummy gate 117 and the fins 106 remain substantially unchanged. In the present embodiment, the epitaxial feature 106 c protects the other layers of the fins 106 when the isolation layer 108 is etched. Subsequently, the epitaxial feature 106 c is removed in another etching process (FIGS. 2O and 3E), exposing the epitaxial feature 106 b for subsequent fabrication stages. The fins 106 may undergo various cleaning and drying processes after the isolation layer 108 and the epitaxial feature 106 c are etched. For example, the fins 106 may be cleaned using a wet cleaning solution and then dried using a spin drying process. Since the fins 106 are supported by the dummy gate 117, they do not suffer from buckling and collapsing issues discussed earlier in the present disclosure. In fact, the fins 106 are supported by either the isolation layer 108 (FIGS. 2H and 3A) or the dummy gate 117 (FIGS. 2N-2O and 3D-3E) throughout the formation of the fins 106 in the channel and S/D regions. This advantageously improves the quality of the fins 106 over existing replacement gate processes.

Furthermore, in embodiments of the present disclosure, the isolation layer 108 is etched in the channel regions 116 and in the S/D regions 122 separately. For example, the isolation layer 108 is etched in the channel regions 116 in operation 24 (FIGS. 1A and 2H) and is etched in the S/D regions 122 in operation 30 (FIGS. 1B, 2N, and 3D). Still further, in embodiments of the present disclosure, the isolation layer 108 is etched in the S/D regions 122 separately for P-type transistors and for N-type transistors. For example, the device 100 may be masked in regions for P-type transistors while the isolation layer 108 is etched in the S/D regions 122 for N-type transistors, and vice versa. The ability of etching the isolation layer 108 in selective regions (channel regions or S/D regions, for P-type transistors or N-type transistors) provides flexibility to the manufacturing process and many advantages to the device 100, which will be discussed in conjunction with FIGS. 4, 5A, and 5B in a later section.

At operation 32, the method 10 (FIG. 1B) optionally forms gate spacers 124 on sidewalls of the dummy gate 117 (FIG. 2P). For example, when the dummy gate 117 includes the poly silicon layer 118, the gate spacers 124 may be formed on sidewalls of the dummy gate 117 to protect the dummy gate 117 during various etching processes. In one example, the fins 106 in the S/D regions are etched to form recesses where epitaxial S/D features are subsequently grown. The poly silicon layer 118 may not have enough etching selectivity with respect to the fins 106. In such a case, the gate spacers 124 (e.g., a nitride) may act as a protection wall for the poly silicon layer 118 during the etching of the fins 106. In an embodiment, the gate spacers 124 may include silicon nitride and may be formed by a deposition and anisotropic etching (e.g., drying etching) process. In an embodiment, the isolation layer 108 may be etched deeper in the S/D regions 122 than in the channel regions 116 during operation 30. To further this embodiment, the gate spacers 124 extend deeper into the isolation layer 108 than the dummy gate 117. When the dummy gate 117 is replaced by a metal gate in a later step, the gate spacers 124 may more effectively prevent the metal material of the metal gate from intruding into the S/D regions 122 than a gate spacer that has the same depth as the dummy gate 117.

At operation 34, the method 10 (FIG. 1B) forms S/D features 125 over the fins 106 in the S/D regions 122. Referring to FIG. 3F, the S/D features 125 may be formed by one or more epitaxial growth processes. The S/D features 125 may include one or more semiconductor materials and may be heavily doped for reducing S/D contact resistance. Further, the method 10 may form S/D features 125 separately for P-type transistors and for N-type transistors. In an embodiment, the fins 106 may be recessed before the S/D features 125 are grown.

After the S/D features 125 are formed, the method 10 may form a contact etch stop (CES) layer covering the various portions of the device 100 and form an inter-layer dielectric (ILD) layer over the CES layer. The CES layer may include a dielectric material such as silicon nitride, silicon oxide, silicon oxynitride, and/or other materials. The CES layer may be formed by ALD, PECVD, or other suitable deposition or oxidation processes. The ILD layer may include materials such as tetraethylorthosilicate oxide, un-doped silicate glass, or doped silicon oxide such as borophosphosilicate glass, fused silica glass, phosphosilicate glass, boron doped silicon glass, and/or other suitable dielectric materials. The ILD layer may be deposited by a PECVD process, a flowable CVD (FCVD) process, or other suitable deposition technique.

At operation 36, the method 10 (FIG. 1B) replaces the dummy gate 117 with a final gate stack. In an embodiment, the final gate stack includes one or more metal layers, and is therefore referred to as a metal gate. For example, operation 36 may remove the dummy gate 117 using one or more etching processes and cleaning processes to form trenches in the device 100. The trenches expose the upper portions of the fins 106 in the channel regions 116 (see FIGS. 2H and 3A). Then, layers of the metal gate are deposited into the trenches and engaging the upper portions of the fins 106. In one example, the metal gate includes an interfacial layer, a gate dielectric layer, a work function metal layer, and a metal fill layer. The interfacial layer may include a dielectric material such as silicon oxide layer (SiO₂) or silicon oxynitride (SiON), and may be formed by chemical oxidation, thermal oxidation, ALD, CVD, and/or other suitable techniques. The gate dielectric layer may include a high-k dielectric layer such as hafnium oxide (HfO₂), zirconium oxide (ZrO₂), lanthanum oxide (La₂O₃), titanium oxide (TiO₂), yttrium oxide (Y₂O₃), strontium titanate (SrTiO₃), other suitable metal-oxides, or combinations thereof. The gate dielectric layer may be formed by ALD and/or other suitable methods. The work function metal layer may be a p-type or an n-type work function layer. The p-type work function layer may comprise titanium nitride (TiN), tantalum nitride (TaN), ruthenium (Ru), molybdenum (Mo), tungsten (W), platinum (Pt), or combinations thereof. The n-type work function layer may comprise titanium (Ti), aluminum (Al), tantalum carbide (TaC), tantalum carbide nitride (TaCN), tantalum silicon nitride (TaSiN), or combinations thereof. The work function metal layer may include a plurality of layers and may be deposited by CVD, PVD, and/or other suitable process. The metal fill layer may include aluminum (Al), tungsten (W), cobalt (Co), copper (Cu), and/or other suitable materials. The metal fill layer may be formed by CVD, PVD, plating, and/or other suitable processes.

At operation 38, the method 10 (FIG. 1B) proceeds to further steps to manufacture the device 100. For example, operation 38 may form S/D contacts electrically contacting the S/D features 125, form gate contacts electrically contacting the metal gate, and form metal interconnects connecting various active (e.g., transistors) and passive (e.g., resistors) devices of the device 100 to form a complete IC.

FIG. 4 illustrates a semiconductor device 200 that is fabricated using embodiments of the method 10. Referring to FIG. 4, the device 200 includes a device region 126 and non-device regions 128. Transistors are formed in the device region 126, but not in the non-device regions 128. Many respects of the device 200 are the same as or similar to the device 100. For example, the device 200 includes a substrate 102, fins 106, an isolation layer 108, and S/D features 125 over the fins 106. In the device region 126, the fins 106 extend vertically (along the “z” direction) from the substrate 102 and through the isolation layer 108, and extend horizontally (in the “x-y” plane, along the “y” direction) through a channel region 116 and two S/D regions 122 of the device 200. A gate stack 130, which may be a metal gate, engages the fins 106 in the channel region 116. FIG. 4 further shows that the isolation layer 108 is lower in the device region 126 than in the non-device region 128. This may result from operations 24 and 30 (FIGS. 1A and 1B) where the isolation layer 108 is etched deeper in the channel and S/D regions of the device 200 and is etched shallower or not etched in the non-device regions 128. FIG. 4 further shows that a CES layer 132 is formed over the isolation layer 108 and an ILD layer 134 is formed over the CES layer 132.

FIGS. 5A and 5B show another device 300 that includes a P-type FINFET 300P and an N-type FINFET 300N. The P-type FINFET 300P and the N-type FINFET 300N may be embodiments of the device 200 (FIG. 4). In FIG. 5A, cross-sectional views of the P-type FINFET 300P and the N-type FINFET 300N, along the “1-1” line of FIG. 4 in the respective channel regions 116, are placed side-by-side for comparison purposes. In FIG. 5B, cross-sectional views of the P-type FINFET 300P and the N-type FINFET 300N, along the “2-2” line of FIG. 4 in the respective S/D regions 122, are placed side-by-side for comparison purposes.

Referring to FIG. 5A, the P-type FINFET 300P is formed in P-type device region 126P and includes P-type fins 106P, the N-type FINFET 300N is formed in N-type device region 126N and includes N-type fins 106N. The P-type device region 126P and the N-type device region 126N are separated by one or more non-device regions 128. The fins 106P and 106N have the same fin height FH in respective channel regions. In an embodiment, the fin height FH is 50 nanometer or higher. Further, the isolation layer 108 has the same height in the regions 126P, 126N, and 128.

Referring to FIG. 5B, the isolation layer 108 has different heights in the regions 126P, 126N, and 128, which may result from operation 30 (FIG. 1B). The isolation layer 108 is higher in the non-device regions 128 than in the device regions 126P and 126N. Accordingly, S/D features 125P of the P-type FINFET 300P and S/D features 125N of the N-type FINFET 300N may be grown to different heights, SH_(P) and SH_(N), respectively. The ability of separately tuning the heights of the S/D features for P-type and N-type transistors enables current spreading for certain fin heights.

Although not intended to be limiting, one or more embodiments of the present disclosure provide many benefits to a semiconductor device and a formation process thereof. For example, semiconductor fins are supported and protected during the formation of the fin (channel and S/D) in replacement gate process. This prevents the fins from buckling and collapsing, especially for fins with high aspect ratio. For another example, fin isolation layer may be etched to different depths in device regions versus non-device regions, and in P-type device regions versus N-type device regions. This provides flexibility of tuning the height of the fin S/D features.

In one exemplary aspect, the present disclosure is directed to a method of forming a semiconductor device. The method includes etching a substrate, thereby forming two first trenches separated by a fin; filling the two first trenches with an isolation layer; and depositing a dielectric layer over the fin and the isolation layer. The method further includes forming a second trench in the dielectric layer over a channel region of the semiconductor device, the second trench exposing the isolation layer. The method further includes etching the isolation layer through the second trench, thereby exposing an upper portion of the fin in the channel region of the semiconductor device; and forming a dummy gate in the second trench over the isolation layer and engaging the upper portion of the fin.

In another exemplary aspect, the present disclosure is directed to a method of forming a semiconductor device. The method includes etching a substrate to form first trenches interposed by fins; filling the first trenches with an isolation layer; depositing a dielectric layer over the fins and the isolation layer; and etching the dielectric layer, thereby forming second trenches in the dielectric layer over channel regions of the semiconductor device. The second trenches expose the isolation layer. The method further includes etching the isolation layer through the second trenches, thereby exposing upper portions of the fins in the channel regions of the semiconductor device. The method further includes forming dummy gates in the second trenches over the isolation layer, the dummy gates engaging the upper portions of the fins. The method further includes removing the dielectric layer and recessing the isolation layer in source/drain regions of the semiconductor device, while the dummy gates cover the channel regions of the semiconductor device.

In yet another exemplary aspect, the present disclosure is directed to a semiconductor device. The semiconductor device includes a device region and a non-device region. The device region includes a channel region and source/drain (S/D) regions for transistors. The semiconductor device comprises a substrate; an isolation layer over the substrate; and a fin element in the device region. The fin element extends vertically from the substrate and through the isolation layer, and extends horizontally in the channel and S/D regions. The semiconductor device further comprises a gate stack engaging the fin element in the channel region. The isolation layer is lower in the device region than in the non-device region.

The foregoing outlines features of several embodiments so that those of ordinary skill in the art may better understand the aspects of the present disclosure. Those of ordinary skill in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those of ordinary skill in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure. 

What is claimed is:
 1. A method of forming a semiconductor device, the method comprising: etching a substrate, thereby forming two first trenches separated by a fin; filling the two first trenches with an isolation layer; depositing a dielectric layer over the fin and the isolation layer; forming a second trench in the dielectric layer over a channel region of the semiconductor device, the second trench exposing the isolation layer; etching the isolation layer through the second trench, thereby exposing an upper portion of the fin in the channel region of the semiconductor device; and forming a dummy gate in the second trench over the isolation layer and engaging the upper portion of the fin.
 2. The method of claim 1, further comprising, after the forming of the dummy gate: removing the dielectric layer from at least source/drain (S/D) regions of the semiconductor device; and etching the isolation layer in the S/D regions of the semiconductor device.
 3. The method of claim 1, wherein the forming of the dummy gate includes: filling the second trench with a poly silicon layer; recessing the poly silicon layer in the second trench such that a top surface of the poly silicon layer is below a top surface of the dielectric layer and is above a top surface of the isolation layer; and forming a hard mask layer over the poly silicon layer in the second trench, wherein the dummy gate includes the poly silicon layer and the hard mask layer.
 4. The method of claim 3, wherein the hard mask layer comprises a nitride.
 5. The method of claim 3, wherein the forming of the hard mask layer includes: depositing the hard mask layer in the second trench and over the dielectric layer; and performing a chemical mechanical planarization (CMP) process to the hard mask layer to expose the dielectric layer.
 6. The method of claim 3, further comprising: removing the dielectric layer from at least source/drain (S/D) regions of the semiconductor device; etching the isolation layer in the S/D regions of the semiconductor device, thereby exposing the fin in the S/D regions of the semiconductor device; and forming a gate spacer on sidewalls of the dummy gate.
 7. The method of claim 1, further comprising: removing the dielectric layer before the forming of the dummy gate, wherein the forming of the dummy gate includes: depositing a hard mask layer in the second trench and over the isolation layer; and performing a CMP process to the hard mask layer to expose the isolation layer, wherein the dummy gate includes a remaining portion of the hard mask layer.
 8. The method of claim 7, wherein the hard mask layer comprises a nitride.
 9. The method of claim 1, further comprising, before the depositing of the dielectric layer: recessing the fin, thereby forming a third trench with the isolation layer being sidewalls of the third trench; and epitaxially growing one or more semiconductor layers in the third trench, wherein the upper portion of the fin includes the one or more semiconductor layers.
 10. The method of claim 1, further comprising: replacing the dummy gate with a metal gate.
 11. A method of forming a semiconductor device, the method comprising: etching a substrate to form first trenches interposed by fins; filling the first trenches with an isolation layer; depositing a dielectric layer over the fins and the isolation layer; etching the dielectric layer, thereby forming second trenches in the dielectric layer over channel regions of the semiconductor device, the second trenches exposing the isolation layer; etching the isolation layer through the second trenches, thereby exposing upper portions of the fins in the channel regions of the semiconductor device; forming dummy gates in the second trenches over the isolation layer, the dummy gates engaging the upper portions of the fins; removing the dielectric layer; and recessing the isolation layer in source/drain regions of the semiconductor device, while the dummy gates cover the channel regions of the semiconductor device.
 12. The method of claim 11, wherein the forming of the dummy gates includes: filling the second trenches with a poly silicon layer; recessing the poly silicon layer in the second trenches such that a top surface of the poly silicon layer is below a top surface of the dielectric layer and is above a top surface of the isolation layer; and forming a hard mask layer over the poly silicon layer in the second trenches, wherein the dummy gates include the poly silicon layer and the hard mask layer.
 13. The method of claim 12, wherein the hard mask layer comprises a nitride.
 14. The method of claim 12, further comprising: forming gate spacers on sidewalls of the dummy gates.
 15. The method of claim 11, wherein the forming of the dummy gates is performed after the removing of the dielectric layer, and wherein the forming of the dummy gates includes: depositing a nitride layer in the second trenches and over the isolation layer; and performing a chemical mechanical planarization (CMP) process to the nitride layer to expose the isolation layer, wherein the dummy gates include a remaining portion of the nitride layer.
 16. The method of claim 11, further comprising: replacing the dummy gates with final gates.
 17. The method of claim 11, further comprising, before the depositing of the dielectric layer: recessing the fins, thereby forming third trenches with the isolation layer being sidewalls of the third trenches; and epitaxially growing one or more semiconductor layers in the third trenches, wherein the upper portions of the fins include the one or more semiconductor layers.
 18. A method comprising: forming a fin structure on a semiconductor substrate; forming an isolation material layer around the fin structure such that the fin structure is embedded in the isolation material layer; forming a dielectric layer over the isolation material layer and the fin structure; removing a first portion of the dielectric layer and a first portion of the isolation material layer to expose a first portion of the fin structure; forming a material layer on the first portion of the fin structure; removing a second portion of dielectric layer and a second portion of the isolation material layer to expose a second portion of the fin structure; forming a source/drain feature over the second portion of the fin structure; removing the material layer over the first portion of the fin structure; and after removing the material layer over the first portion of the fin structure, forming a gate dielectric layer and a gate electrode layer over the first portion of the fin structure.
 19. The method of claim 18, wherein the material layer includes a polysilicon material.
 20. The method of claim 18, further comprising forming a hard mask layer over the material layer on the first fin structure; and removing the hard mask layer over the first portion of the fin structure after forming the source/drain feature over the second portion of the fin structure. 